Windows within windows within windows...

We have developed an application which lets us talk to custom electronics. It’s an MDI application, and there are a number of windows in the app: an editor, a runwindow, temperature sensors, and a plot window.

The plot window contains a number of pages. Inside each page, you can define any number of plots. These can be dragged and dropped, resized, etc. There can be up to fifty plots like that, divided over 5-15 pages.

So the structure looks as follows:
Main Window -> Plot MDI window -> Tabs -> Frame with slidebar -> Plots that can be resized

Basically, we’ve created a windowing system inside the MDI-subwindow for plotting. Users are happy, but it works non-intuitively and instead of further maintaining the code, we’d like to use standard Qt widgets for the plot resizing. We’ve looked at QDockWidget, but it can only be used right below a Main Window. Any ideas?